Wireless network solutions are currently being installed at an increasing rate to follow the demand from the market for faster data communication connections and wider wireless coverage of data and voice communication. This demand comes from the fact that the number of services available in public networks, such as the Internet, increases and the contents of many services has a high bandwidth demand. For instance, services such as music and video streaming acquire a high portion of available bandwidth, Voice over IP (VoIP) applications also sets up a demand for higher bandwidth than pure text messages or html data traffic. Voice and VoIP applications also need a certain level of Quality of Service (QoS) in order to reduce latency problems which will degrade the experienced quality of communication. The human is very sensitive to this kind of latency problem.
When setting up the infrastructure for such wireless communication networks, care and consideration must be taken in order to obtain efficient coverage. Many problems may arise in building infrastructure installations since radio communication may be affected by many different disturbances, such as obstacles, other radio sources, moving communication devices, and so on. This is true for fixed installation and even more important for semi fixed or ad hoc network infrastructure installations where it is extremely difficult to predict how infrastructure nodes will interact with each other and with client stations.
In a wireless network with several client stations, sharing the media through time-division, it is important to reduce the number of collisions occurring when multiple stations try to communicate with an infrastructure node, such as a so called Access Point (AP) or base station. One scheme for reducing this risk has been developed for fixed wired data networks and is called CSMA (carrier sense multiple access) with a collision detection (CD) system; however, it is very difficult not to say impossible to implement the CD scheme efficiently in a wireless network without using two radio interfaces operating simultaneously. Therefore, in a wireless network another system is often used: a Collision Avoidance (CA) CSMA system. A client station in such a network must before transmitting traffic, sense the medium to determine if another client station is transmitting. If so, the sensing client station must wait for a minimum specified duration plus a random number of time before initiating a transmission again. However, again the client station must sense the medium for ongoing traffic before transmitting signals.
One problem not solved by the CSMA/CA system in wireless networks is the so called hidden node problem, wherein two stations try to communicate with an infrastructure node, such as a so called access point (AP) or base station, or with another client station in for instance an ad-hoc based network at the same time and they due to network topology considerations can not detect communication from each other. This may occur for instance when two client stations are located on opposite sides of an infrastructure node in such a way that radio signals can not reach from one client station to another, or two client stations both try to communicate with another client station located in such a way that the first two client stations do not detect each others transmissions.
A scheme solving the hidden node problem is the so called RTS/CTS scheme (Request to Send/Clear to Send), wherein nodes communicating with each other must exchange control packets in order to establish a connection before transmitting actual data packets. However, this solution introduces a large overhead in control traffic slowing down communication links and increases the radio transmission overhead.
A crucial parameter in wireless networks for the above mentioned situations is the transmission power level. The power level affects the range of signals, power consumption in devices, interference between nodes, and so on and is thus important to control in a wireless network system. However, a crude power control aiming only at reducing the power levels used, may lead to an increase of hidden node problems.
Several different attempts to control transmission power for both clients and infrastructure nodes have been developed previously for certain applications. In some wireless protocol solutions regulatory demands has been raised in order to be able to set the maximum allowed transmit power in a network cell. This is true for the IEEE 802.11h protocol standard which is an amendment of the IEEE 802.11a protocol standard in Europe. However, in this case only the maximum allowed transmission level is adjustable in order to allow for regulatory demands, this problem do not address the problem of reducing the power consumption for devices or the hidden node problem.
Several authors have addressed the power consumption problem and may be found described in different documents. For instance in WO03003657 Soomro et al disclose a method and apparatus for adjusting the transmission power level by determining a signal to noise margin for the communication link of interest. This scheme is however limited to addressing such problems as of limiting the transmitter power in general, for battery consumption purposes, avoiding radio interference. It does not try to remedy the problem of the hidden node situation discussed above.
US 2003/0100328 discloses adjustment of the power level for both mobile stations (STA) and an access point (AP) in order to minimize radio coverage of an AP in order to reduce interference problems between access points in an area of high AP density. However, this invention only deals with signalling of transmit power control levels to mobile stations and not how to determine the power levels.
In WO9907105 again a system for using optimum transmission levels in a wireless multihop network is disclosed. This applies for the situation when stations communicate with each other and the system takes different quality disturbing parameters into account, such as path loss, phase distortion, time delay, Doppler shift, and multipath fading, all concerned with radio characteristics. However, the invention does not take hidden node problems into account when determining the optimum path for a data packet.
In WO0057575 a path loss estimate is calculated from received power levels during transmission and continued communication power levels are based on this estimate. A similar function is found in WO02091623 but with a slightly different path loss estimation function.
In U.S. Pat. No. 5,553,316 an invention is disclosed that adjusts the transmit power level in order to use only the needed power level for obtaining a reliable transmission. Again, the hidden node problem is not solved. However, a defer threshold value is introduced that limits a sending station to withholding communication when a carrier signal above the defer threshold value is sensed by the sender.
In US patent application 2004/0029590 the hidden node problem is addressed by transmitting data traffic from an access point at a lower power level reducing the risk for the interference problems since thus a lower number of stations can hear the transmission. However, in order to make stations outside the communication area aware of the network, control traffic (such as RTS/CTS/ACK) is transmitted using a higher power level. This system does not address the problem of determining appropriate power levels to avoid hidden node problems when said control traffic, that cause significant overhead, is not used.